Refactor to fix warnings around frozen strings

[CloCkWeRX]

When running tests, there were a number of warnings about modifying a frozen string.

Turns out, we can just rewrite this to be a ActiveRecord::SafeBuffer, which is safe to mutate.

@CloCkWeRX ➜ /workspaces/dynamic_form (html-safe) $ bundle exec rake test
Loaded suite /usr/local/rvm/gems/ruby-3.4.1/gems/rake-13.3.0/lib/rake/rake_test_loader
Started
Finished in 0.083011199 seconds.
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31 tests, 44 assertions, 0 failures, 0 errors, 0 pendings, 0 omissions, 0 notifications
100% passed
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373.44 tests/s, 530.05 assertions/s